Apparatus as described above are in clinical use, and typical examples thereof are disclosed for example, in DE-A-33 19 871, DE-A-27 22 252 and DE-B-31 19 295.
In such apparatus, focussed ultrasonic shock waves are aimed at a concretion, that is to say a solid mass of foreign material, present within an organ of the body, to destroy the concretion, thereby removing it by a non-surgical method. After such treatment only a fine grit remains in said organ, which is for example a kidney, and such grit is flushed from the body naturally.
Before triggering a firing sequence of comparatively powerful ultrasonic waves, the precise location of the concretion which is to be destroyed must be established. An A scanner and/or a B scanner are commonly used for this purpose.
A problem that occurs during the use of such apparatus is that healthy tissue adjacent to the concretion may be damaged by the action of the ultrasonic shock waves, if the focus of the ultrasonic transmission system does not coincide with the concretion or tissue (where said object is tissue) to be destroyed.
The accuracy required in positioning the focus with respect to the object to be destroyed, for example a kidney stone, depends among other things upon the principle according to which the ultrasonic waves are generated. If, for example, the shock waves are generated according to the known submerged arc discharge principle and are focussed by means of ellipsoidal reflectors, the focal point diameters typically so obtained are of the order of magnitude of approximately 10 mms. Since such a focal point diameter is large in relation to a stone of the usual size, no great precision in positioning the focus is then needed.
If, however, the ultrasonic shock waves are generated by means of better focussed sources of shock waves, for example by means of piezoelectric transducers according to DE-A-33 19 871, focal point diameters are obtained which only have an order of magnitude of approximately 2 mms. The ratio between the size of the stone and the focal point diameter is, therefore, smaller than where the shock waves are generated according to said arc discharge principle as discussed above. Precise positioning of the focus is, therefore, needed in order to prevent injury to the tissue surrounding the stone where a sequence of shock wave pulses is triggered at an instant when the focus does not coincide with the stone.
Even where such a small diameter focus has been positioned so as to coincide with a stone to be destroyed, it cannot be assumed that the stone will remain in its initial position throughout the ultrasonic treatment.
The concretion, in this case the stone, which is to be destroyed can alter its position under the influence of cyclically recurrent bodily functions and in particular as a result of the patient's respiration, and the position of the stone may also be altered by the initial ultrasonic bombardment of the stone.
Thus, the triggering of sonic pulse sequences should be avoided in such a case, where the concretion to be destroyed leaves the originally correctly positioned focus of the ultrasonic transmitter as a result of a bodily function. DE-A-36 21 935, for example, discloses a triggering system which controls the firing sequence as a function of a bodily activity, that is to say as a function of a comparison between a threshold value and an actual value.
Shock waves may be applied not only for the destruction of concretions, but also for sealing off blood vessels for example by a clotting action, or for the direct destruction of unhealthy tissue such as tumours, according to DE-A-35 44 344. Substantially the same problem that occurs in the destruction of concretions also arises in these other cases, namely that care should be taken to ensure that adjacent healthy tissue is not impaired by the ultrasonic shock waves. Triggering systems which are controlled by a bodily function, are, however, applicable only where said function is recurrent, and are, therefore, for controlling the release of shock waves as a function of an isolated event, for example the shifting of an object to be destroyed in the body after an initial ultrasonic bombardment, or the movement of the patient on the operating table.